A new Trump administration regulation unveiled Monday would make it more difficult for legal immigrants who may qualify for public benefits to obtain citizenship -- and likely would limit legal immigration.

Politically, it could further sharpen the contrast between Republicans and Democrats on an issue the president has put at the heart of his reelection bid.

As the administration framed the new policy as an effort to discourage immigrants' reliance on taxpayer-funded government programs, several Democrats running for president criticized the new rule, set to go in effect in October, and accused the administration of targeting poorer immigrants.

Leah Millis/Reuters

President Donald Trump talks to reporters as he departs for travel to New York and New Jersey from the South Lawn of the White House in Washington, Aug. 9, 2019.

"It's just an ongoing campaign of his to vilify a group of people," Sen. Kamala Harris told CNN Monday.

After nearly the entire Democratic field voiced support for providing health care to undocumented immigrants in a recent debate, Trump tweeted, "How about taking care of American Citizens first!?"

The Iowa State Fair's "Cast Your Kernel" tent features a decidedly unscientific survey of potential caucusgoers -- they drop an unpopped kernel into a mason jar with the name of the candidate -- but its results through Monday still closely tracked with many legitimate polls -- Joe Biden, Elizabeth Warren, Pete Buttigieg, Kamala Harris and Bernie Sanders lead the Democratic field.

What stands out, however, when voters are queried about their support for the former vice president, is that few name any policies he champions, instead falling back on the idea of electability.

"When you think about who's electible … he's the one who stands out for me," said Liz Vaccariello, a fair attendee who summed up the feelings of many Biden-backers ABC News spoke to over the weekend.

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People walk past a food stand at the Iowa State Fair, Aug. 11, 2019, in Des Moines, Iowa.

"I try to put my hat on that says, 'OK, say I'm a middle-America voter and I hear Elizabeth Warren and Bernie Sanders talk and I think that's exactly what's not going to get elected.'"

But while this narrative has clearly taken hold among some voters, it doesn't exactly jive with recent polls, which frequently show Warren, and especially Sanders, also topping Trump in head-to-head matchups.

And that data is beginning to emerge on the trail, where supporters of the Vermont senator have begun to break out into chants of "Bernie beats Trump" at his rallies.

With more than five months until the Iowa Caucus, there is plenty of time for the field to seize on the questionable concept that only Biden is safely electable, and when that time comes, without a signature policy to tout as an alternative pitch, will the former vice president, as Warren would say, "Have a plan for that?"

THE PLAYLIST

ABC News' "Start Here" podcast. Tuesday morning's episode features ABC News Senior Investigative Reporter Aaron Katersky, who brings us up to speed on where the Jeffrey Epstein case goes from here after his death in prison. Then ABC News' Serena Marshall explains the new rules for legal immigrants seeking public assistance. http://apple.co/2HPocUL .

WHAT YOU NEED TO KNOW TODAY

President Donald Trump tours Shell Pennsylvania Petrochemicals in Monaca, Pennsylvania, and then speaks at 2:10 p.m.

South Bend, Indiana, Mayor Pete Buttigieg delivers his pitch to Iowa State fairgoers at the Des Moines Register's Political Soapbox at 1:30 p.m. CDT before touring the fairgrounds.

Sen. Bernie Sanders, I-Vt., holds a town hall in Berlin, New Hampshire, at 10 a.m. and later hosts an ice cream social at 1 p.m. in Littleton.

Sen. Elizabeth Warren, D-Mass., participates in a Facebook Live Q&A with members of the Working Families Party at 6 p.m. EDT.

Former Secretary of Housing and Urban Development Julian Castro participates in an affordable housing town hall in Des Moines, Iowa, at 1 p.m. CDT and then travels to Marshalltown, Iowa, for a meet-and-greet at 6:30 p.m.

Former Rep. Joe Sestak, D-Pa., delivers remarks to the Cedar County Democrats at 6:30 p.m. CDT in Tipton, Iowa, and then travels to Washington, Iowa, to speak to Washington County Democrats at 7:30 p.m.

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The Note is a daily ABC News feature that highlights the key political moments of the day ahead. Please check back tomorrow for the latest.